fixed undesirable side-effects

... simply by creating a new Tiddler object

kudos to @cdent for helping me figure out I was being stupid, looking in
all the wrong places and getting myself confused along the way:
https://github.com/FND/tiddlywebplugins.gitstore/commit/6033f94afb4068203acd2775502016b1870c5d81#commitcomment-2466842
thus now using py.test's `-x` switch to avoid drawing wrong conclusions
in the future